Timing generator

ABSTRACT

A delay setting data generator generates delay setting data based on rate data. A variable delay circuit delays the test pattern data by a delay time determined by the delay setting data with reference to a predefined unit amount of delay. First rate data designates the period of the test pattern data with a precision determined by the unit amount of delay. Second rate data designates the period of the test pattern data with a precision higher than that determined by the unit amount of delay. The delay setting data generator outputs a first value and a second value in a time division manner at a ratio determined by the second rate data, the first and second values being determined by the first rate data.

CLAIM OF PRIORITY

This application is a continuation application under 35 USC 120 andclaims priority from co-pending U.S. application Ser. No. 12/344,424,filed on Dec. 26, 2008, both of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technology for testing semiconductordevices and, more particularly, to a technology of controlling theperiod (test rate) of a test pattern.

2. Description of the Related Art

A test apparatus is used to supply a test pattern to a device under test(DUT) so as to determine whether the device is defective or not byinspecting its operation. A pattern generator (PG) for generating a testpattern to be supplied to a DUT and a timing generator (TG) for definingthe timing of supplying a test pattern to a DUT are installed in a testapparatus. The period (frequency) of a test pattern is alternativelyreferred to as a test rate. A test apparatus is required to provide thefunction of altering a test rate arbitrarily.

Timing generators are broadly categorized into two types according tothe method of operation.

The first method uses a phase lock loop (PLL). Specifically, a PLLcircuit is used to multiply a reference clock and the timing of a testpattern is controlled in synchronization with the multiplied signal. ThePLL method allows setting a test rate arbitrarily by switching thefrequency division ratio of the PLL circuit. Hereinafter, this methodwill also be referred to as the PLL method.

The second method uses a variable delay circuit. This method achieves anarbitrary test rate by setting the amount of delay of a variable delaycircuit according to a test rate and delay the test pattern accordingly.Alternatively a variable delay circuit introduces an arbitrary delay ina set signal and a reset signal defining the timing of transition of atest pattern so that a test pattern is caused to make a transition insynchronization with the set signal and the reset signal thus delayed.This method will also be referred to as a phase accumulation (PA)method.

Semiconductor devices available today operate at increasingly higherspeeds. Higher speeds of devices mean higher test rates. In the nearfuture, a test apparatus will be required to control a test rate with anextremely high resolution on the order of sub-picoseconds (ps).

For high resolution control of a test rate in a PLL-based timinggenerator, either (1) the pulse swallow method or (2) ΔΣ fractionalN-PLL method is employed. The pulse swallow method has an advantage inthat the design is easy but has a disadvantage in that there are somefrequency division ratios that cannot be set (i.e., test rates thatcannot be set). Meanwhile, the ΔΣ fractional N-PLL method has anadvantage in that arbitrary frequency division ratios (test rates) canbe achieved but has a disadvantage in that fractional spurious isgenerated. Providing a ΔΣ noise shaper to eliminate fractional spuriouswill cause another problem in that phase noise is increased. Stillanother problem is that spurious signals will be increased at a specificfrequency division ratio.

A test apparatus targeting memory devices and some non-memory devices issometimes required to change the test rate from moment to moment on areal time basis. Such a control is sometimes referred to as real timetiming control (RTTC) or on the fly control. In principle, on the flycontrol using a PLL-based timing generator is impossible because of asettling time (lock up time) required to elapse until the PLL producesoscillation at a preset period.

Meanwhile, switching of the test rate in a PA-based timing generator isnone other than switching of the amount of delay of the variable delaycircuit. Since the switching of the amount of delay of the variabledelay circuit only requires a very short time, the PA method is employedfor on the fly control of the test rate.

The resolution of the test rate of the PA-based timing generatorcorresponds to the resolution of the amount of delay of the variabledelay circuit. Variable delay circuits currently available provide aresolution of one to several ps. To obtain a higher resolution (e.g.,sub ps), a dramatic increase in hardware scale would be required or itwould be almost impossible to design such a circuit.

SUMMARY OF THE INVENTION

In this background, an illustrative purpose of the present invention isto provide a technology capable of controlling the test rate of a timinggenerator with a high resolution.

One embodiment of the present invention relates to a timing generatoradapted to receive test pattern data to be supplied to a device undertest and rate data for setting the period of the test pattern data, andcontrol the timing of outputting the test pattern data to the deviceunder test according to the rate data. The timing generator comprises: adelay setting data generator operative to receive the rate data andgenerate delay setting data; and a variable delay circuit operative todelay the test pattern data by a delay time determined by the delaysetting data with reference to a predefined unit amount of delay. Therate data comprises first rate data designating the period of the testpattern data with a precision determined by the unit amount of delay andsecond rate data designating the period of the test pattern data with aprecision higher than that determined by the unit amount of delay. Thedelay setting data generator is operative to output a first value and asecond value in a time division manner at a ratio determined by thesecond rate data, the first value being determined by the first ratedata and the second value being determined by the first rate data anddifferent from the first value.

The phrase “delay the test pattern” encompasses delaying at least one ofa signal indicating a positive edge of the test pattern and a signalindicating a negative edge of the test pattern as well as delaying thetest pattern itself. The phrase broadly encompasses any measures fordelaying a signal that affects the timing of the test pattern data.

According to this embodiment, the test rate can be controlled with aresolution higher than the resolution of the variable delay circuit, byusing the first and second values and the frequencies of theiroccurrence.

The delay setting data generator may generate a serial data sequence inwhich 0s and 1s occur at frequencies determined by the second rate data,add the bits of the serial data sequence to the first rate data orsubtract the bits of the serial data sequence from the first rate data,and output the resultant data as the delay setting data.

In this case, the first and second values may be set to the first ratedata itself and data derived from adding 1 to the first rate data. Theratio of the first and second values may be set according to the secondrate.

The delay setting data generator may include a pseudorandom datagenerator capable of controlling the mark-to-space ratio according tothe second rate data, and use the output from the pseudorandom datagenerator as the serial data sequence.

In this case, deviation of the test rate over time is reduced by usingthe pseudorandom data generator.

The delay setting data generator may include an nth-order ΔΣ modulatorfor subjecting the second rate data to ΔΣ modulation, where n is anatural number, add the sequence of ith-order bits of the second ratedata D_(RATE2) thus modulated to the ith bits in the lower group of bitsin the first rate data, respectively, or subtract the ith-order bitsfrom the ith bits, respectively, where 1≦i≦n, and outputs the resultantdata as the delay setting data.

In this case, deviation of the test rate over time is further reduced.

The delay setting data generator may further comprise an integratoroperable to integrate the first rate data at intervals defined by afirst clock. Given that division of the data output from the integratorby a reference value determined by the period of the first clockproduces a quotient α and a remainder β, where α and β denote integers,the delay setting data generator may set the first value and the secondvalue to values determined by the remainder β. The timing generator maydelay the test pattern data by a total of α periods of the first clock.

Another embodiment of the present invention relates to a test apparatus.The test apparatus comprises: a pattern generator operative to producetest pattern data to be supplied to a device under test; and the timinggenerator according to any of the embodiments described above operableto control the timing of outputting the test pattern data to the deviceunder test according to rate data.

Still another embodiment of the present invention relates to a method ofcontrolling a test rate at which test pattern data is output to a deviceunder test, based on rate data for setting the period of the testpattern data to be supplied to the device under test. The rate datacomprises first rate data designating the period of the test patterndata with a precision determined by the unit amount of delay and secondrate data designating the period of the test pattern data with aprecision higher than that determined by the unit amount of delay. Themethod comprises: setting delay setting data to be a first value and asecond value in a time division manner at a ratio determined by thesecond rate data, the first value being determined by the first ratedata and the second value being determined by the first rate data anddifferent from the first value; and delaying the test pattern data by adelay time determined by the delay setting data with reference to apredefined unit amount of delay.

It is to be noted that any arbitrary combination or rearrangement of theabove-described structural components and so forth is effective as andencompassed by the present embodiments. Moreover, this summary of theinvention does not necessarily describe all necessary features so thatthe invention may also be a sub-combination of these described features.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings which are meant to be exemplary,not limiting, and wherein like elements are numbered alike in severalFigures, in which:

FIG. 1 shows the structure of a test apparatus according to anembodiment;

FIG. 2 shows an exemplary structure of the timing generator of FIG. 1;and

FIGS. 3A-3C show exemplary structures of a modulator.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described based on preferred embodiments whichdo not intend to limit the scope of the present invention but exemplifythe invention. All of the features and the combinations thereofdescribed in the embodiment are not necessarily essential to theinvention.

In the present specification, the state represented by the phrase “themember A is connected to the member B” includes a state in which themember A is indirectly connected to the member B via another member thatdoes not affect the electric connection therebetween, in addition to astate in which the member A is physically and directly connected to themember B. In the same way, the state represented by the phrase “themember C is provided between the member A and the member B” includes astate in which the member A is indirectly connected to the member C, orthe member B is indirectly connected to the member C via another memberthat does not affect the electric connection therebetween, in additionto a state in which the member A is directly connected to the member C,or the member B is directly connected to the member C.

FIG. 1 shows the structure of a test apparatus 100 according to anembodiment. The test apparatus 100 is provided with a pattern generator(PG) 1, a timing generator (TG) 2, a form shaper (FC) 3, a driver 5, acomparator 6, and a determination unit 7.

The pattern generator 1 generates test pattern data D_(PAT) that shouldbe supplied to a DUT 200. The test pattern data D_(PAT), when outputfrom the pattern generator 1 as parallel data, is converted into aserial bit series by a data serializer.

The test apparatus 100 is provided with the function of arbitrarilysetting the period of the test pattern data D_(PAT) according to acommand or program set by the user. The timing generator 2 receives thetest pattern data D_(PAT) and rate data D_(RATE) for setting the periodof the test pattern data D_(PAT). The timing generator 2 controls thetiming of outputting the test pattern data D_(PAT) to the DUT accordingto the rate data D_(RATE).

The form shaper 3 receives the test pattern data D_(PAT) subjected totiming control by the timing generator 2 and sets the data D_(PAT) tohave a data format suitable for the DUT 200. The form shaper 3 is alsoreferred to as a format controller (FC) because of its function.

The driver 5 supplies the test pattern output from the form shaper 3 tothe DUT 200. For example, in case the DUT 200 is a memory, the testpattern is written in a designated address. The test pattern, oncewritten, is read out again. If the DUT 200 is not defective, the writtenpattern and the read pattern should match. The level of the pattern readfrom the memory is determined by the comparator 6 so as to producedevice data D_(DUT). The pattern generator 1 generates expectation dataD_(EXP) such that the timing of generation is set by the user. Thedetermination unit 7 determines whether the device data D_(DUT) and theexpectation data D_(EXP) match, identifies non-defective DUT 200, andmay identify a defective location.

Described above is the overall structure of the test apparatus 100. Adetailed description will now be given of the structure of the timinggenerator 2 according to the embodiment.

The timing generator 2 is of phase accumulation type and is primarilyprovided with a delay setting data generator 10 and a variable delaycircuit 30. The delay setting data generator 10 receives the rate dataD_(RATE) and generates delay setting data D_(DS).

The variable delay circuit 30 is a delay circuit operating withreference to a predefined unit amount of delay τu. For example, thecircuit 30 includes a plurality of unit delay elements (buffers) incascade connection. The structure of the variable delay circuit 30 isnot limited to the one described above. Various delay circuits capableof digital control of the amount of delay may be used.

The variable delay circuit 30 delays the test pattern data D_(PAT) by adelay time τ defined by the delay setting data D_(DS) with reference tothe unit amount of delay τu. For example, the amount of delay τ of thevariable delay circuit 30 is set to a value derived by multiplying theunit amount of delay τu by the delay setting data D_(DS). The unitamount of delay τu is between one to several ps. In the followingdescription, it is assumed that τu=1 ps for ease of understanding andbrevity.

While FIGS. 1 and 2 illustrate the variable delay circuit 30 as delayingthe test pattern data D_(PAT) itself, this does not limit the scope ofthe present invention. For example, the variable delay circuit 30 mayequivalently delay edge signals (also referred to as a set signal and areset signal) that designate the timing of edges (a positive edge and anegative edge) of level transition of the test pattern data D_(PAT). Inthis case, an RS flip-flop set and reset by a set signal and a resetsignal, respectively, is provided in addition to the variable delaycircuit 30. Any publicly known technology may be employed to achievesuch a structure in the present invention.

The rate data D_(RATE) fed to the timing generator 2 includes first ratedata D_(RATE1) and second rate data D_(RATE2). The first rate dataD_(RATE1) designates the period of the test pattern D_(PAT) (test rate)with a precision of the unit amount of delay τu. The second rate dataD_(RATE2) designates the period of the test pattern with a precisionhigher than the unit amount of delay τu, i.e., with a sub-picosecondresolution τf. The first rate data D_(RATE1) and the second rate dataD_(RATE2) may be a higher group of bits and a lower group of bits,respectively, which form a series of bits D_(RATE). Alternatively,D_(RATE1) and D_(RAM) may be separate data.

The delay setting data generator 10 sets the delay setting data D_(DS)to one of the first value X1 and the second value X2. The first value X1is determined by the first rate data D_(RATE1). The second value X2 isdetermined by the first rate data D_(RATE1) and is different from thefirst value X1.

The first value X1 and second value X2 may be set in any appropriatemanner. For example, the values may be set according to expressions (1)and (2) below.

X1=D _(RATE1)+δ1  (1)

X2=D _(RATE1)+δ2  (1)

where δ1 and δ2 are different integers and may be zero or negativevalues.

The delay setting data generator 10 outputs, as the delay setting dataD_(DS), the first value X and second value X2 in a time-division mannerat a ratio (Y1:Y2) determined by the second rate data D_(RATE2). Y1denotes the statistical probability that the delay setting data D_(DS)takes the first value X1, and Y2 denotes the statistical probabilitythat the data D_(DS) takes the second value X2, where the followingexpression holds.

Y1+Y2=1  (3)

In this case, the amount of delay applied by the variable delay circuit30 to the test pattern data D_(PAT) as average over time will be givenby the following expression.

τ=τu×(X1×Y1+X2×Y2)  (4)

Substituting expressions (1)-(3) into expression (4), we obtain thefollowing expression.

$\begin{matrix}\begin{matrix}{\tau = {\tau \; u \times \left\{ {{\left( {D_{{RATE}\; 1} + {\delta \; 1}} \right) \times Y\; 1} + {\left( {D_{{RATE}\; 1} + {\delta \; 2}} \right) \times Y\; 2}} \right\}}} \\{= {{\tau \; u \times D_{{RATE}\; 1} \times \left( {{Y\; 1} + {Y\; 2}} \right)} + {\tau \; u \times \left( {{\delta \; 1 \times Y\; 1} + {\delta \; 2 \times Y\; 2}} \right)}}} \\{= {{\tau \; u \times D_{{RATE}\; 1}} + {\tau \; u \times \left( {{\delta \; 1 \times Y\; 1} + {\delta \; 2 \times Y\; 2}} \right)}}}\end{matrix} & (5)\end{matrix}$

For example, given that δ1=0 and δ2=1, expression (5) may be written asfollows.

τ=τu×D _(RATE1) −τu×Y2  (5a)

For example, to obtain a test rate τ=10.1 ps, the following setting maybe used.

D_(RATE1)=10

τu=1 ps

Y2=0.1

Described above is the structure and operating principle of the timinggenerator 2 according to the embodiment. The value of the second term(δ1×Y1+δ2×Y2) on the right side of expression (5) may be a non-integralvalue, i.e., may be a fraction or decimal fraction. The value varies inaccordance with the second rate data D_(RATE2). Accordingly, the timinggenerator 2 according to the embodiment is capable of controlling theamount of delay τ given by expression (5) with a resolution higher thanthe unit amount of delay τu.

A description will now be given of a specific, exemplary structure ofthe timing generator 2. FIG. 2 is a circuit diagram showing an exemplarystructure of the timing generator of FIG. 1.

The timing generator 2 is provided with a delay setting data generator10, a variable delay circuit 30, a first multiplexer 42, a secondmultiplexer 44, a second AND gate 46, and a frequency multiplier 48. Thetiming generator 2 comprises two blocks including a logic unit 2 aoperating in synchronization with the first clock LREFCLK having thefirst frequency f1 and an analog unit 2 b operating in synchronizationwith the second clock HREFCLK having a frequency higher than the firstfrequency f1.

For example, a reference clock REFCLK from an external source is used asthe first clock LREFCLK. The second clock HREFCLK is generated bymultiplying the reference clock REFCLK using the frequency multiplier48. The frequency multiplier 48 may be a PLL circuit or DLL (delaylocked loop). Hereinafter, it will be assumed that f2=8×f1 for brevityand ease of understanding.

As described above, the timing generator 2 uses the PA method.

The delay setting data generator 10 is provided with a modulator 12, anintegrator 14, a second adder 20, and a counter 22.

The integrator 14, the second adder 20, and the counter 22 perform asignal process according to the PA method. The integrator 14 integratesthe first rate data D_(RATE1) at intervals defined by the first clockLREFCLK.

For example, the integrator 14 includes a first adder 16 and a delaycircuit 18. The delay circuit 18 delays the data output from the firstadder 16 by the period of the first clock LREFCLK. The first adder 16adds the first rate data D_(RATE1) and the data output from the firstadder 16 delayed by a single period.

More specifically, given that the value 4 is continuously input as thefirst rate data D_(RATE1), the output D_(RATE1)′ of the integrator 14will be increased such that 4, 8, 12, 16, . . . . In practice, signalprocessing is performed using the binary format instead of decimalnumbers.

The second adder 20 adds the data output from the integrator 14 anddelay data D_(DELAY). The delay data D_(DELAY) is set so as to delay thedata output to the DUT by a predefined period of time irrespective ofthe test rate.

The second adder 20 outputs data D_(RATE3) (hereinafter, referred to ascumulative data) determined by the first rate data D_(RATE1) asintegrated over time.

The cumulative data D_(RATE3) is divided by a reference value T1determined by the period of the first clock LREFCLK so as to produce aquotient α and a remainder β.

Preferably, the reference value T1 is a factorial of 2, i.e., T1=2^(p),where p denotes a natural number. In this case, a divider is notnecessary since division is equivalent to a bit shift. The lower p bitsof the cumulative data D_(RATE3) represent a remainder and the remaininghigher bits represent a quotient α. A divider may be providedseparately.

The quotient data α is fed to the counter 22. The counter 22 counts thefirst clock LREFCLK and raises a gate signal G1 to a high level eachtime the count reaches α. A first AND gate 40 uses the gate signal G1 togate the test pattern data D_(PAT). This process delays the test patterndata D_(PAT) by a total of α periods of the first clock LREFCLK.

The remainder data β is fed to the modulator 12. The remainder data β isdata determined by the first rate data D_(RATE1).

The modulator 12 outputs, as the delay setting data D_(DS), the firstvalue X1 determined by the remainder β (the first rate data D_(RATE1))and the second value X2 determined by the remainder data β in atime-division manner at a ratio determined by the second rate dataD_(RATE2). Such a process can be understood as a sort of modulation.

The first multiplexer 42 subjects the delay setting data D_(DS) outputfrom the modulator 12 to parallel-to-serial conversion. Similarly, thesecond multiplexer 44 subjects the data output from the first AND gate40 to parallel-to-serial conversion. The second AND gate 46 gates(re-times) the data output from the second multiplexer 44 according tothe second clock HREFCLK. The variable delay circuit 30 delays the testpattern data D_(PAT)′ output from the second AND gate 46 according tothe delay setting data D_(DS)′ output from the first multiplexer 42.

A description will now be given of the modulator 12. FIGS. 3A-3C show anexemplary structure of the modulator 12.

A modulator 12 a of FIG. 3A is provided with a high resolution datagenerator 50 and a selector 52. The first value X1 (=β+δ1) and thesecond value X2 (=β+δ2) are fed to the selector 52. The high resolutiondata generator 50 generates serial data (hereinafter, referred to ashigh resolution data D_(F)) where 1s and 0s occur at the respectivefrequencies determined by the second rate data D_(RATE2). The selector52 the selects the first value X1 when the high resolution data D_(F) isat 0 and selects the second value X2 when the high resolution data D_(F)is at 1. The selector 52 outputs the resultant data as the delay settingdata D_(DS). It should be noted that the probability that the delaysetting data D_(DS1) takes the value 1 is none other than theprobability that the delay setting data D_(DS) takes the second valueX2.

The modulator 12 b of FIG. 3B includes the high resolution datagenerator 50 and a third adder 54. The modulator 12 b may be suitablyused when δ1=0. The high resolution data generator 50 generates highresolution data D_(F). The third adder 54 adds the remainder data β andthe high resolution data D_(F) and outputs the resultant data as thedelay setting data D_(DS). A subtractor may be used in place of thethird adder 54. In this case, δ2 is set to a negative value.

The high resolution data D_(F) may be a bit stream in which 1 bit occursat a time. In this case, signal processing when δ1=0 and δ2=1 issuitably performed.

Referring to FIGS. 3A and 3B, the high resolution data generator 50preferably comprises a pseudorandom data (PRBS) generator capable ofcontrolling the mark-to-space ratio according to the second rate dataD_(RATE2). By using the pseudorandom data PRBS as the high resolutiondata D_(F), deviation of the test rate over time is reduced.

The high resolution data D_(F) generated by the modulator 12 b may be ann-bit parallel bit stream, where n is a natural number. In this case, δ1is set such that δ1=0 and δ2 can be set to an arbitrary value in therange 0<δ2<2^(n).

In the modulator 12 c of FIG. 3C, the high resolution data D_(F) is ann-bit parallel bit stream, where n is a natural number. The highresolution data generator 50 includes an nth-order ΔΣ modulator 62. Thenth-order ΔΣ modulator 62 subjects the second rate data D_(RATE2) to ΔΣmodulation so as to generate the n-bit parallel high resolution dataD_(F). The third adder 54 adds the high resolution data D_(F) and theremainder data β (the first rate data D_(RATE1)) or subtracts the highresolution data D_(F) from the remainder data β. The modulator 12 cgenerates the delay setting data D_(DS) by adding the sequence ofith-order (1≦i≦n) bits of the second rate data D_(RATE2) thus modulatedto the corresponding ith bits of the remainder data β (i.e., the ithbits in the lower group of bits in the first rate data D_(RATE1)).

For example, in case a second-order ΔΣ modulator (n=2) is used, the highresolution data D_(F) output from the n-th order ΔΣ modulator 62 takesone of the values [00], [01], [10], [11]. The probability of occurrenceof the values is set according to the second rate data D_(RATE2).According to this structure, deviation of the test rate over time issuitably reduced further.

Described above is the structure of the timing generator. A descriptionwill now be given of the operation.

For ease of understanding, it will be assumed that δ1=0, δ2=1, X1=β,X2=β+1, and Y2=D_(RATE2)/Z1. Z1 may be an arbitrary constant number. Ifthe second rate data D_(RATE2) comprises m bits (m is a natural number),Z1 may be such that Z1=2^(m). In this case, the resolution τf of thetest rate based on the second rate data D_(RATE2) is given byτf=τu/2^(m). The highest bit of the second rate data D_(RATE2) isweighted by a factor τ/2, the second highest bit τ/4, . . . , and thelowest bit τ/2^(m).

For example, given that τ=1 ps and m=3, the test rate can be controlledwith a resolution of τf=0.125 ps. Given that τu=1 ps, m may be set suchthat m=1 (bit) if the resolution of 0.5 ps is required. If theresolution of 0.25 ps is required, m may be set such that m=2 (bits).

The pseudorandom data generator 60 of FIG. 3A or FIG. 3B deals withsuccessive 8 (=2^(m)) bits as one frame and asserts 1 in a total of(D_(RATE2)) bits. For example, given that the first rate dataD_(RATE1)=[00011] and the second rate data D_(RATE2)=[011], Y2=⅜ andY1=1−Y2=⅝. Therefore, the high resolution data D_(F) will be random datasuch as [01010100] and [10010001], where a mark (“1”) occurs in 3 of the8 bits.

Thus, according to the timing generator 2 of the embodiment, the testrate can be set to a fractional value by switching the value of thedelay setting data D_(DS) in a time division manner. Switching of thetest rate can be made on the fly at every 1 bit (symbol) of the testpattern.

By configuring the timing generator 2 as shown in FIG. 2, thearchitecture of the related-art timing generator can be inherited sothat the load imposed on a circuit designer is significantly reduced.The additional circuitry is concentrated in the logic unit 2 a so thatfull logic implementation is possible. The related-art circuit may beused as it is for the analog unit 2 b.

The test rate set by the timing generator 2 takes a central valuedetermined by the rate data D_(RATE) when viewed macroscopically over along time span. On a microscopic scale, the rate exhibits a randomfluctuation. In other words, it looks as if jitter is superimposed onthe rate. It should be noted that two components are included in thejitter. More specifically, the first component is derived from jitter inthe first clock LREFCLK and the second clock HREFCLK (i.e., jitterinherent in the timing generator), and the second component isfluctuation of the test rate superimposed as a result of transition ofthe delay setting data D_(DS). It is possible to configure the secondcomponent to be substantially equal to or sufficiently smaller than thefirst component. This means that the test rate can be controlled with ahigh resolution in a manner that the fluctuation is embedded in thejitter component of the clock inherent in the timing generator 2. Itshould therefore be appreciated that fractional delay control isguaranteed not to cause adverse effects on test results.

While the preferred embodiments of the present invention have beendescribed using specific terms, such description is for illustrativepurposes only, and it is to be understood that changes and variationsmay be made without departing from the spirit or scope of the appendedclaims.

1. A timing generator adapted to receive test pattern data to besupplied to a device under test and rate data for setting the period ofthe test pattern data, and control the timing of outputting the testpattern data to the device under test according to the rate data, therate data D_(RATE) having a first rate data D_(RATE1) that is a integerpart of D_(RATE) and a second rate data D_(RATE2) that is a fractionalpart of D_(RATE), the timing generator comprising: a delay setting datagenerator operative to receive the rate data and generate delay settingdata D_(DS) according to the rate data D_(RATE); and a variable delaycircuit operative to delay the test pattern data by a delay time τdetermined by the delay setting data D_(DS) with reference to apredefined unit amount of delay τu, τ=τu×D_(DS), wherein, the delaysetting data generator is operative to output the delay setting dataD_(DS) having a first value X1, X1=D_(RATE1)+δ1 and a second valueX2=D_(RATE1)+δ2 in a time division manner at a ratio Y1:Y2, δ1 and δ2being constant integers and the ratio Y1:Y2 being determined accordingto the second rate data D_(RATE2), and Y1 (0<Y1<1) denotes thestatistical probability that the delay setting data D_(DS) takes thefirst value X1, and Y2 denotes the statistical probability that thedelay setting data D_(DS) takes the second value X2, and Y1+Y2=1 ishold, and wherein, the delay setting unit is operative to determine Y1and Y2 such that a relation (1) is established,X1×Y1+X2×Y2=D _(RATE1) +D _(RATE2)  (1).
 2. The timing generatoraccording to claim 1, wherein the delay setting data generator generatesa serial data sequence in which 0s and 1s occur at frequenciesdetermined by the second rate data, adds the bits of the serial datasequence to the first rate data or subtracts the bits of the serial datasequence from the first rate data, and outputs the resultant data as thedelay setting data.
 3. The timing generator according to claim 2,wherein the delay setting data generator includes a pseudorandom datagenerator capable of controlling the mark-to-space ratio according tothe second rate data, and uses the output from the pseudorandom datagenerator as the serial data sequence.
 4. The timing generator accordingto claim 1, wherein the delay setting data generator includes annth-order ΔΣ modulator for subjecting the second rate data to ΔΣmodulation, where n is a natural number, adds the sequence of ith-orderbits of the second rate data thus modulated to the ith bits in the lowergroup of bits in the first rate data, respectively, or subtracts theith-order bits from the ith bits, respectively, where 1≦i≦n, and outputsthe resultant data as the delay setting data.
 5. The timing generatoraccording to claim 1, wherein the delay setting data generator furthercomprises an integrator operable to integrate the first rate data atintervals defined by a first clock, given that division of the dataoutput from the integrator by a reference value determined by the periodof the first clock produces a quotient α and a remainder β, where α andβ denote integers, the delay setting data generator sets the first valueand the second value to values determined by the remainder β, and thetiming generator delays the test pattern data by a total of α periods ofthe first clock.
 6. A test apparatus comprising: a pattern generatoroperative to produce test pattern data to be supplied to a device undertest; and the timing generator according to claim 1 operable to controlthe timing of outputting the test pattern data to the device under testaccording to rate data.
 7. A method of controlling a test rate at whichtest pattern data is output to a device under test, based on rate datafor setting the period of the test pattern data to be supplied to thedevice under test, wherein the rate data comprises first rate datadesignating the period of the test pattern data with a precisiondetermined by the unit amount of delay and second rate data designatingthe period of the test pattern data with a precision higher than thatdetermined by the unit amount of delay, and the method comprises:setting delay setting data to be a first value and a second value in atime division manner at a ratio determined by the second rate data, thefirst value being determined by the first rate data and the second valuebeing determined by the first rate data and different from the firstvalue; and delaying the test pattern data by a delay time determined bythe delay setting data with reference to a predefined unit amount ofdelay.
 8. The method according to claim 7, wherein the setting of thedelay setting data comprises: generating a serial data sequence in which0s and 1s occur at frequencies determined by the second rate data; andadding the bits of the serial data sequence to the first rate data orsubtracting the bits of the serial data sequence from the first ratedata, so as to set the delay setting data to be the data resulting fromaddition or subtraction.
 9. The method according to claim 7, wherein thesetting of the delay setting data comprises: subjecting the second ratedata to n-th order ΔΣ modulation, where n is a natural number, andadding the sequence of ith-order bits of the second rate data thusmodulated to the ith bits of the first rate data, respectively, orsubtracts the ith-order bits from the i-th bits, respectively, where1≦i≦n.